Direct sequence CDMA (DS-CDMA) is particularly useful in multiple-access communications because it allows efficient use of the frequency spectrum and provides for improved frequency reuse. However, DS-CDMA systems can suffer from interference and distortion that reduce their ability to support data communications.
OFDM has a high spectral efficiency (the spectrum of the subcarriers overlap) and combats frequency-selective fading. However, the amplitude of each carrier is affected by the Rayleigh law, hence flat fading occurs. Even with good channel estimation and channel coding, fading and interference can easily compromise the performance of OFDM.
Multi-carrier CDMA (MC-CDMA) applies CDMA spreading codes to orthogonal subcarriers to enhance frequency-diversity benefits. However, MC-CDMA, like OFDM, suffers from a high peak-to-average-power ratio (PAPR). The high PAPR of conventional multi-carrier signals imposes significant constraints on the transmission circuitry and significantly increases power consumption.
Carrier Interferometry (CI) is a multi-carrier technology invented by Applicant and described in over 70 technical journals and conference proceedings, as well as in two textbooks. As a multicarrier transmission protocol, CI provides unsurpassed performance and versatility compared to all other technologies.
The description of the preferred embodiments assumes that the reader has a familiarity with CI, such as described in the following publications, which are incorporated by reference:    1. B. Natarajan, C. R. Nassar, S. Shattil, M. Michelini, “Application of interferometry to MC-CDMA”, accepted for publication in IEEE Transactions on Vehicular Technology.    2. C. R Nassar, B. Natarajan, and S. Shattil, “Introduction of carrier interference to spread spectrum multiple access,” IEEE Emerging Technologies Symposium, Dallas, Texas, 12-13 April 1999.    3. B. Natarajan and C. R. Nassar, “Introducing novel FDD and FDM in MC-CDMA to enhance performance,” IEEE Radio and Wireless Conference, Denver, Colo., Sep. 10-13, 2000, pp. 29-32.    4. Z. Wu, C. R. Nassar, A. Alagar, and S. Shattil, “Wireless communication system architecture and physical layer design for airport surface management,” 2000 IEEE Vehicular Technology Conference, Boston, Mass., Sep. 24-28, 2000, pp. 1950-1955.    5. S. Shattil, A. Alagar, Z. Wu and C. R. Nassar, “Wireless communication system design for airport surface management—Part I: Airport ramp measurements at 5.8 GHz,” 2000 IEEE International Conference on Communications, Jun. 18-22, 2000, New Orleans, pp. 1552-1556.    6. B. Natarajan, C. R. Nassar, and S. Shattil, “Carrier Interferometry TDMA for future generation wireless—Part I: Performance,” accepted for publication in IEEE Communications Letters.    7. Z. Wu, C. R. Nassar, and S. Shattil, “Capacity enhanced DS-CDMA via carrier interferometry chip shaping,” IEEE 3G Wireless.Symposium, May 30-Jun. 2, 2001, San Francisco, Calif.    8. Z. Wu, C. R. Nassar, and S. Shattil, “Frequency diversity performance enhancement in DS-CDMA via carrier interference pulse shaping,” The 13th Annual International Conference on Wireless Communications, Calgary, Alberta, Canada, Jul. 7-10, 2001.    9. C. R. Nassar and Z. Wu, “High performance broadband DS-CDMA via carrier interferometry chip shaping,” 2000 International Symposium on Advanced Radio Technologies, Boulder, Colo., Sep. 6-8, 2000.    10. Z. Wu and C. R. Nassar, “MMSE frequency combining for CI/DS-CDMA,” IEEE Radio and Wireless Conference, Denver, Colo., Sep. 10-13, 2000, pp. 103-106.    11. D. Wiegandt, C. R. Nassar, and S. Shattil, “High Performance OFDM for next generation wireless via the application of carrier interferometry,” IEEE 3G Wireless Symposium, May 30-Jun. 2, 2001, San Francisco, Calif.    12. B. Natarajan, C. R. Nassar, and S. Shattil, “Exploiting frequency diversity in TDMA through carrier interferometry,” Wireless 2000: The 12th Annual International Conference on Wireless Communications, Calgary, Alberta, Canada, Jul. 10-12, 2000, pp. 469-476.    13. S. A. Zekevat, C. R. Nassar, and S. Shattil, “Smart antenna spatial sweeping for combined directionality and transmit diversity,” accepted for publication in Journal of Communication Networks: Special Issue on Adaptive Antennas for Wireless Communications.    14. S. A. Zekevat, C. R. Nassar, and S. Shattil, “Combined directionality and transmit diversity via smart antenna spatial sweeping,” 38th Annual Allerton Conference on Communications, Control, and Computing, Champaign-Urbana, Ill., Oct. 4-6, 2000.    15. S. Shattil and C. R. Nassar, “Array Control Systems For Multicarrier Protocols Using a Frequency-Shifted Feedback Cavity” IEEE Radio and Wireless Conference, Denver, Colo., Aug. 1-4, 1999.    16. C. R. Nassar, et. al., MultiCarrier Technologies for Next Generation Multiple Access, Kluwer Academic Publishers: 2001.    17. U.S. patent application entitled “Carrier Interferometry Networks,” filed May 14, 2002.    18. U.S. patent application entitled “Multicarrier Sub-Layer for Direct-Sequence Channel and Multiple-Access Coding,” filed Apr. 24, 2002.    19. PCT Appl. PCT/US01/50856 entitled “Carrier Interferometry Coding and Multicarrier Processing,” filed Dec. 26, 2001.    20. U.S. Pat. application entitled “Multiple Input, Multiple Output Carrier Interferometry Architecture,” filed Nov. 22, 2000.    21. U.S. patent application entitled “Method and Apparatus for Transmitting and Receiving Signals having a Carrier Interferometry Architecture,” filed Jul. 19, 2000.    22. U.S. patent application entitled “Method and Apparatus for using Multicarrier Interferometry to Enhance Optical Fiber Communications,” filed Nov. 2, 1999.    23. PCT Appl. PCT/US99/02838 entitled “Multiple Access System and Method,” filed Feb. 10, 1999.
Many implementations of CI codes are characterized by block coding. In single-carrier modulation, pulse shaping provides sequential overlapping pulse waveforms. Even block transmissions, such as direct-sequence codes and other time-domain sequences (e.g., packets, frames, etc.) are more accurately characterized by sequential pulse waveforms than by block-coded CI pulse waveforms. Time-domain equalization, Rake reception, and other time-domain receiver processes can sometimes provide poorer performance results when block-coded CI waveforms are used rather than sequential waveforms. Similarly, block frequency-domain processing of sequential waveforms can provide poorer performance than processing block waveforms.
Thus, there is a need in the art to provide CI transceivers with the capability of efficiently processing both block and sequential types of signal waveforms. There is also a need in the art to efficiently process single-carrier signals via multi-carrier techniques. This need is naturally extended to multi-antenna processing wherein greater bandwidth efficiencies are achieved by employing frequency-domain or time-domain processing together with spatial processing.